1. Field of the Invention
The present invention concerns a method to determine the measurement (data acquisition) workflow of a magnetic resonance tomography apparatus in the generation of slice images of a subject. The invention furthermore concerns a device for controlling a magnetic resonance tomography apparatus.
2. Description of the Prior Art
Magnetic resonance tomography (also called magnetic resonance imaging, MRI) is an imaging method for depiction of structures inside subjects, essentially inside bodies. Slice images of the subject to be examined (the human or animal body) can be generated with magnetic resonance tomography, which slice images permit a comparison and an orientation of anatomical slices of the same region and allow an assessment of the organs and many organ variations. Magnetic resonance tomography uses magnetic fields and radio-frequency electromagnetic waves. The basis for the image contrast is the different sensitivity (susceptibility) of the examined tissue to the applied physical variables.
Nuclear magnetic resonance forms the physical basis of magnetic resonance tomography. Protons as well as neutrons have an inherent angular momentum (spin), and charged particles thereby receive a magnetic moment in a magnetic field. If a rotating atomic nucleus is brought into a static magnetic field, it is aligned according to the field. The rotation axis of the nucleus rotates in the direction of the applied magnetic field. A precessional movement occurs any time when the nucleus is brought out of its rest position. If the external field is removed, the nucleus falls back into its original position. If a second field (what is known as a transverse field) is applied which is perpendicular to the static field, the nucleus begins to precess again until an equilibrium state arises. This is likewise the case when the field is switched off again. In order to intentionally excite nuclei is an examination to precession, this second field is an alternating radio-frequency field and rotates in an x-y plane.
A resonant frequency exists for the precession movement of the nuclear spin. For atomic nuclei, this eigenfrequency is called the Larmor frequency. It depends on the strength of the applied magnetic field and on the structure of the nucleus. Which nuclei are resonated can be very precisely determined by the selection of the strength of the first static field and the selection of the frequency of the transversal field. The macroscopic magnetic moment of the nucleus is tilted by 90° in to the x-y plane due to the resonance effect and rotates precessing with the transversal field.
If the transverse alternating field which has tilted the magnetic moment of a nucleus by 90° is deactivated, the nucleus rotates further in the x-y plane. If a coil is brought into proximity to the rotating magnetic moment, a voltage is induced in this call. Typically the measurement coils normally are in the x-y plane, and the measured voltage is proportional to the transversal magnetization of the magnetic moment of the atomic nucleus. A rotating transverse magnetization arising from the transverse magnetizations of the individual nuclei can be generated with a series of radio-frequency pulses of the transverse field in a subject that lies in a strong magnetic field. This transverse magnetization is dependent on the location and on the tissue type of the subject to be examined.
The goal of magnetic resonance tomography is the generation of slice images of the transverse magnetization. The use of magnetic resonance tomography apparatuses is associated with high costs for purchasing and maintenance. The development of new magnetic resonance tomography apparatuses therefore seeks to reduce the operating costs in addition to the improvement of technical aspects. One of the possible solutions is to improve the time efficiency of magnetic resonance tomography. This means that it is sought to reduce the time for generation of the slice images of a subject.
For this purpose, methods known as turbo spin echo sequences (TurboSE) have been developed that achieves an acceleration of the measurement workflow by the optimization of the known spin echo and gradient echo techniques. An acquisition matrix thus can be generated with higher speed, but contrast losses in the slice image generation must be accepted. The acquisition matrix represents a total number of measurement regions that are necessary for generation of a slice image. A method called a “turbo inversion time recovery sequences (TurboIRTSE)” is also known that represents a development of TurboSE. This exhibits the advantage that it can generate a much stronger contrast in the slice images and suppresses tissue (such as liquid or fat) in the depiction. A disadvantage of TurboIRTSE is that an additional inversion radio frequency (IRF) pulse is required which makes the method less time-efficient.